The present invention relates to an acousto-optic device particularly in apparatus suitable for deflecting light beams and for information processing, wherein the diffraction of light is obtained on application of acoustic waves in an optically anisotropic medium of a single crystal of univalent mercury halide.
Existing acousto-optic devices for deflecting light beams and for optical information processing operate on the principle of the diffraction of light wave by the application of acoustic waves induced in a suitable acousto-optic medium by a piezoelectric transducer. By changing the frequency of the acoustic wave one deflection angle of the light beam is changed. By changing the amplitude of the acoustic wave the intensity of the deflected beam is controlled. The diffraction of the light beam, on application of the acoustic wave can also result in a change of the polarization of the light wave. Depending upon the application of these devices in systems used for deflecting the light beams (e.g. deflectors) and in systems used for information processing the most important parameters of the acousto-optic device are the diffraction efficiency, i.e. the ratio between the intensities of the deflected and incident beam, and the product of the time constant .tau. of the device with the width .DELTA.f of the frequency band. This bandwidth is determined on one hand by the electric and acoustic properties of the piezoelectric transducer and on the other hand by the interaction bandwidth of the acoustic and light wave. An extremely large bandwidth of the acousto-optic interaction can be attained in an optically anisotropic medium by making use of the so-called abnormal diffraction, in which the polarization of the deflected light beam changes. In an optically uniaxial media the transverse acoustic wave is introduced, as a rule, in a direction parallel or perpendicular to the optical axis, whereas the direction of the incident beam is chosen so as to have the deflected beam emerging perpendicular to the optical axis.
In using the crystal types known so far, the described arrangement leads to rather high acoustic frequencies in the gigahertz range and the diffraction efficiency is low.
In the crystal of paratellurite, strong rotation of the polarization plane of light can be utilized for the construction of an acousto-optic device with abnormal diffraction and high diffraction efficiency. The operating frequency of such acousto-optic unit is relatively low, in the range of tens of megahertz, depending on the wavelength of light employed. In this case, however, the incident light must have approximately circular polarization.
An acousto-optic delfector utilizing the abnormal diffraction in a rotated tellurium dioxide crystal is also known. The acoustic wave in the tellurium dioxide crystal propagates in the direction inclined at an angle of 6.degree. from the [110] axis in the (110) plane, with the [110] direction of vibrations. This arrangement retains the high efficiency of interaction and removes the decrease of the diffraction efficiency in the middle of the frequency band. A disadvantage of this deflector is that the group velocity direction of the acoustic wave is inclined from the wave normal at a large angle, 51.3.degree. nominally. Consequently, an extremely large crystal volume is required for the construction of such a deflector. Besides, deflectors made from the tellurium dioxide crystals cannot be used in the infrared spectral range beyond 5 .mu.m. Another disadvantage is the relatively high price for the tellurium dioxide single crystals of the required dimensions and quality. In addition, still another disadvantage of the acousto-optic device made from tellurium dioxide consists in the fact that the acousto-optic quality factor M.sub.2, which determines the diffraction efficiency for the diffraction by longitudinal waves, is rather small--about 1/30 of the value for the diffraction by transverse waves.
An acousto-optic device made from a single crystal of univalent mercury halide, is described by Dobrzhanskii et al. in the CSSR author's certificate No. 170 007, which has high value of the acousto-optic quality factor M.sub.2 for both longitudinal and transverse waves, and which in addition transmits the radiation even in the infrared spectral range with wavelengths larger than 5 .mu.m. A disadvantage of this device however, is that, due to the low propagation velocity of the acoustic wave, sufficient frequency bandwidth can be attained only by using a piezoelectric transducer of very small dimensions, whereby the requirements on the acoustic power density generated by the transducer are very severe.
The above-mentioned disadvantages are avoided in the acousto-optic device according to the present invention.